Video: Kilograms

In this video, we will learn how to use kilogram weights and a scale to measure the mass of objects.

12:20

Video Transcript

Kilograms

In this video, we’re going to learn how to use kilogram weights and a scale to measure the mass of objects. A kilogram is a unit of measurement that we use for measuring mass. You may come across people using the word kilo. This is just a shorter way of saying kilogram. Also, we often use the letters kg to represent kilograms. Saves us writing out the word all the time. Now, what does a kilogram look like? You may’ve seen something like this before. It’s a kilogram weight. It’s made out of metal and it weighs exactly one kilogram. We can use weights like this to help measure the mass of objects.

Do you have an idea how much this would weigh if you picked it up? It’s often useful to have an idea of objects that weigh around about one kilogram. This can help us to visualize or imagine how heavy a kilogram is. Can you imagine picking up a pineapple? This weighs about the same as a kilogram. The sort of bags of rice that you might find in a kitchen cupboard often weigh a kilogram.

It’s also about the same mass as one of those large dictionaries that you can get. You can also get some small laptops that weigh one kilogram too. And here’s a really interesting fact. If we don’t count the bottle, a liter of water weighs exactly the same as one kilogram. Perhaps you have your own way of remembering how heavy one kilogram is. But hopefully these objects may help you.

When we look at the word kilogram, we can see that part of it is the word gram. We can measure the mass of objects using grams too. It’s another unit of measurement. But the kilogram is a heavier unit than grams are for measuring mass. We often measure the mass of more heavy objects in kilograms. So how can we use kilograms to find the mass of objects? Well, there’re two ways really. The first is using what we might call a set of balances or weighing scales. The word balances is interesting here, though, because we need to look at our scales and think about how they are balanced.

At the moment, the two ends of our scale are balanced. There’s nothing on them. They both weigh the same. Oh dear, what’s about to happen now? Our scales aren’t balanced anymore. This is because the bird is heavier than what’s on this end. There’s nothing on this end, is there? Let’s put a kilogram weight on the other end and see what happens. This is interesting. The scales still unbalance, but this time, they’ve changed direction. What can we say about the mass of the bird? We don’t know exactly how heavy it is. But what we do know is that the bird is lighter than a kilogram.

We can tell this because of what’s happened to the set of balances. Let’s take the kilogram weight off for a moment. Oh dear, here we go. Oh this is getting silly. Let’s find out the mass of these three birds before they fall off. The birds are heavier than one kilogram. The scales are now balanced. This time we can say exactly how heavy the birds are. These birds weigh two kilograms.

Now there’s another piece of equipment that we can also use to measure mass in kilograms. These are a set of weighing scales. Can you see there’s a dial on the front of them? This part of the scales is where we’re going to read how heavy things are. If you know how to read a number line like this, you can easily read these scales because the dial on the front is just a curved number line. And on these scales, we can see the numbers from one to nine. And if we look carefully, we can see that the unit of measurement is kilograms.

At the moment, the arrow is pointing to zero because nothing is on the scales. So all we need is something to weigh. Let’s weigh this cactus. We just need to put it on the scales. If we look at where the arrows move to, we can see that it’s halfway between the numbers three and four. We can say that the cactus weighs three and a half kilograms. We found out the weight by reading the scales. Let’s have a go now at answering some questions where we have to use kilogram weights and a scale to measure the mass of some objects.

Are the bananas heavier or lighter than a kilogram?

In the picture, we can see a set of balancing scales. On one end, there’s a bunch of bananas. And at the other end, we have a metal weight. We can see that this is labeled one kg. And this shows us that the unit of measurement is a kilogram. This is a kilogram weight. So what these scales are helping us to do is to compare the masses of these two objects. And this is why the question asks us, are the bananas heavier or lighter than a kilogram?

What do you notice about the position of the scales? This end of the scales is further down, isn’t it? This shows that the kilogram weight is heavier than the bananas. So we can say that the bananas are lighter than the kilogram. We don’t know how heavy the bananas are exactly. But by looking at our set of balances, we can say that the bananas are lighter than a kilogram.

Fill in the blank: The mass of the puppy is what kilograms.

In the picture, we can see a set of balancing scales. This is the sort of equipment we can use to help weigh the mass of different objects. In this case, we’re using them to measure the mass of a puppy. And we can see the puppy at one end of our measuring scales. And at the other end, we have what look like some metal weights. Can you see what’s written on them? Each of the weights is labeled one kg or kilogram. So how many kilograms are at this end of our scales? There are four of them. Each one is worth one kilogram, so we can say that there are four kilograms altogether.

Now our question asks us to fill in the blank with the mass of the puppy. In other words, how heavy does the puppy weigh? Well, we could just look at our kilogram weights and say it weighs four kilograms. And that it’s actually the right answer. But before we say that, we need to think carefully. Why is the mass of the puppy four kilograms? And it’s not just because four kilograms are on the other end of our scales.

These scales have four kilograms at one end and a puppy at the other. But on these scales, the puppy is lower than the four kilograms. If our scales look like this, we would know that the puppy was heavier than four kilograms. It would weigh more than four kilograms. And if our set of scales look like this, again, we’d still have four kilograms on one end and a puppy on the other. But this time, it’s the four kilogram weights that’re heavier. This end of our set of scales is pushed down. The puppy is lighter than four kilograms. And so in this example, we would say that the puppy is less than four kilograms.

Now, let’s come back to our picture. How would you say we know that the mass of the puppy is four kilograms? Yes, there are four kilograms at one end and a puppy at the other. But the important thing is that our scales are balanced; both ends of the scale weigh the same. The mass of the puppy is equal to four kilograms. Can you see how they’re both level with each other? We didn’t just find the answer by counting the number of kilograms. We looked at the fact that our weighing scales were balanced. And we know now that the mass of the puppy is four kilograms.

Emma weighed a pumpkin and a watermelon. Figure out the mass of the pumpkin.

This is a really interesting question. We need to figure out or work out the mass of a pumpkin. And we’re given some pictures to help us. The only trouble is none of the pictures show that Emma has weighed a pumpkin on its own. She’s used this set of balancing scales to weigh a pumpkin and a watermelon together. And then, she’s just used a different set of scales to weigh the mass of a watermelon on its own. We need to use these two pieces of information to work out the mass of what the pumpkin weighs on its own.

First of all, let’s look at the pictures and work out what each set of scales is showing. Once we’ve done that, we can think about how we can work out the answer. Before we start, let’s make sure that we’re measuring in the same unit of measurement. Can you see the letters kg on both sets of scales? We know that kg is short for kilograms. And kilograms are a unit of measurement that we can use to measure the mass of heavier objects like pumpkins or watermelons.

Let’s look at our first set of scales. To begin with, we can see that they’re balanced. In other words, one end isn’t heavier than the other. They both weigh the same. It’s like being on a seesaw where someone on the other end is exactly the same mass as you. So we can use this set of scales to work out how heavy a pumpkin and a watermelon together are. Let’s count how many kilograms there are. We can see three weights on the bottom and two on the top. So that’s five kilogram weights altogether. The pumpkin and the watermelon weigh five kilograms together.

Now let’s read our second set of scales. These have got a dial on the front of them. And there’s a needle that moves. Can you see which number it’s pointing to? By putting the watermelon on these scales, it’s made the needle point to the number two. So we now know how heavy the watermelon is on its own. It’s two kilograms. Now, it’s time to use these two pieces of information to help work out the answer. Let’s use a part–whole model to help us.

We know that the total mass of a pumpkin and a watermelon is five kilograms. And we also know that the mass of a watermelon on its own is two kilograms. Can you see how we can work out the answer? We just need to think what goes with two to make five. We know that five take away two equals three. We didn’t need to weigh the pumpkin on its own. We used the information on both sets of scales to help us. The mass of a pumpkin on its own is three kilograms.

What have we learned in this video? We’ve been introduced to the kilogram and some objects that weight about the same as a kilogram. We’ve also learned how to use kilogram weights and a scale to measure the mass of objects.

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